This invention relates generally to method and apparatus for growing a single crystal and more particularly to a method for growing multiple crystals and apparatus used in the method.
The substantial majority of monocrystalline silicon which is used to make silicon chips for the solid-state electronics industry is produced by the Czochralski method. Briefly described, this method involves melting chunks of high-purity polycrystalline silicon in a quartz crucible located in a specifically designed furnace to form a silicon melt. An inert gas such as argon is typically circulated through the furnace. A relatively small seed crystal is mounted above the crucible on a pulling wire which can raise and lower the seed crystal. The crucible is rotated and the seed crystal is lowered into contact with the molten silicon in the crucible. When the seed begins to melt, it is slowly withdrawn from the molten silicon and starts to grow, drawing silicon from the melt.
To increase the rate of production of single crystal from one furnace, it is desirable to grow more than one crystal from the silicon melt in the crucible before the crucible is replaced with a new crucible containing a fresh batch of silicon. The process of replacing the crucible is time consuming and considerably slows the rate of production of crystals. Although it is possible to add molten silicon to the melt after the crystal is grown or during the growth process, the number of crystals which may be grown from the melt is still limited by the concentration of impurities such as Fe and C in the melt. The concentration of Fe, C and other impurities in the melt is initially in the parts per million and parts per billion range, which is necessary to produce crystals of sufficient purity. The impurities have different affinities for remaining in liquid or going to a solid form in the crystal during the growth process. This affinity is represented numerically by the segregation coefficient of the particular impurity. If the segregation coefficient is less than one, the impurity tends to remain in the liquid as silicon is drawn off by the seed crystal. For example, C has a coefficient of 0.01 and Fe has a coefficient of 8.times.10.sup.-6. Therefore, even if new silicon is added to the melt, the concentration of these impurities quickly grows to unacceptable levels, and the crucible must be replaced with a new crucible containing a fresh batch of molten silicon.